1. Field of the Invention
This invention concerns a capacitive device for measuring dimension.
2. Description of the Related Art
Capacitive measuring devices are used in precision measuring instruments, for example in calipers or in height measuring columns. They generally comprise a scale and a transducer, both equipped with electrodes. The capacitive coupling between the different electrodes changes when the transducer is displaced opposite the scale. The value of this coupling or its evolution is assessed, and allows the position of the transducer to be determined.
It would be possible for the position to be determined with just a single electrode on the transducer being displaced opposite a single electrode on the scale. When the two electrodes are perfectly superimposed, the capacitive coupling is maximal. If the transducer is displaced, the coupling diminishes. The displacement of the transducer can thus be determined by measuring this coupling.
To increase the resolution and to render the system less sensitive to errors, particularly positioning errors, the use of arrays of several electrodes is generally preferred, both on the scale and on the transducer. Depending upon the configuration adopted, the majority of these devices can be grouped into two main types.
The first main type concerns multiple emitting and simple receiving systems. The patent document EP 0 184 584 (Mauser Werke Oberndorf) describes a system of this type. In this system, the transducer bears three groups of emitting electrodes which receive AC signals mutually phase shifted by 120.degree., for example sinusoidal signals. These signals are transmitted capacitively to scale electrodes, then, in return, to two receiving electrodes on the transducer. The signal obtained on these two electrodes is then converted by analyzing means into a result indicating the position of the transducer.
The patent documents U.S. Pat. No. 4,743,902 (Andermo), U.S. Pat. No. 4,878,013 (Andermo), U.S. Pat. No. 4,449,179 (Meyer) and U.S. Pat. No. 4,437,055 (Meyer) describe variant systems using multiple emission of N signals and simple reception on one or two electrodes.
These devices require the generation of N signals, for example N phase-shifted sinusoidal signals. The precision in measurement generally depends upon the precision in phase shift between the N signals obtained. If N is large, and depending upon the shape of the desired signals, design of the N signal generator is difficult and a large surface of integrated circuit is required.
EP 0 622 612, of which the present applicant is the patent holder, describes a variant of this type of system, measuring the change in frequency of signal received rather than its phase for determining the position of the transducer. This system is thus less sensitive to errors in phase. However, this system also requires a complex N signal generator.
The second main type among capacitive measuring devices are the simple emitting and multiple receiving systems. Such a system is disclosed, for example, in the Swiss patent CH 648 929, of which the present applicant is the patent holder. In these systems, a single signal is emitted on a sole emitting electrode. N electrodes or groups of receiving electrodes on the transducer receive N different signals, for example mutually phase-shifted signals. The N signals received are then analyzed to determine the position of the transducer.